The optical axis of a vehicle headlamp may be set in a predetermined direction relative to a road surface to achieve a light distribution characteristic to reduce glare for a vehicle traveling in the opposite direction or a foregoing vehicle. An irradiation direction control apparatus has been proposed for deflecting the optical axis up and down such that a direction of the headlamp's optical axis forms a predetermined angle relative to a road surface even when the elevation angle, or pitch angle, is changed as a result of the vehicle's loading or traveling condition. For example, Japanese patent document JP-A-2003-40029 discloses providing vehicle height sensors at front and rear wheel portions of the automobile and calculating the change in pitch angle using an ECU (electronic control unit) based on a vehicle height of the front and rear wheel portions as detected by the vehicle height sensors. The optical axis of a headlamp is controlled based on the calculated pitch angle.
The foregoing type of irradiation direction control apparatus typically needs to be initialized at a factory for manufacturing the automobile (i.e., an assembly factory). FIG. 5(a) is a conceptual view for explaining the initialization operation. The optical axis of the headlamp (HL) is set to a predetermined reference angle θ0 after bringing the automobile (CAR) to a reference state by setting (stopping) the automobile CAR on a horizontal level block B. At the same time, the heights of vehicle body positions FP, RP at the front and rear wheel portions are detected by vehicle height sensors. The vehicle height of the front wheel portion in the reference state is designated as Hf0 (for example, in units of millimeters (mm)). Similarly, the vehicle height of the rear wheel portion is designated as Hr0 (mm). The vehicle heights are stored in memory within the ECU as reference vehicle heights. A pitch angle θp of the automobile is set as “0”. When the front or rear portion of the automobile is inclined upwards or downwards as the result of a change in a passenger or a load, a traveling condition or the like, the vehicle height of the front or rear wheel portions is changed. The ECU calculates the pitch angle of the automobile and controls the optical axis angle of the headlamp HL based on the calculated pitch angle. For example, if the vehicle height of the front wheel portion becomes Hfx (mm) and the vehicle height of the rear wheel portion becomes Hrx (mm) as shown by FIG. 5(b), the pitch angle relative to the reference state of the automobile (pitch angle θp=0) becomes θpx, and the angle of the headlamp's optical axis becomes θHLx. Hence, the pitch angle θpx is calculated based on amounts of changes in the front and rear vehicle heights ΔHf (=Hfx−Hf0), ΔHr (=Hrx−Hr0). The optical axis of the headlamp is controlled to deflect upwards or downwards such that the optical axis angle θHL of the headlamp serves as the reference optical axis angle θ0 even when the pitch angle θpx is changed. Thus, the irradiating direction can be controlled such that the optical axis of the headlamp HL remains at a constant angle relative to the road surface regardless of a change in the pitch angle.
In initializing the irradiating direction control apparatus described above, the respective vehicle heights of the front and rear wheel portions detected by the vehicle height sensors are stored in memory as the reference vehicle heights. For example, an output voltage (unit: V (volt)) detected by the vehicle height sensor is converted into the vehicle height (unit: mm (millimeter)) by the ECU and is stored in the memory. The structure for attaching the vehicle height sensor differs according to the type of automobile, as shown by FIG. 6. Therefore, a characteristic value in converting the output voltage into the vehicle height differs depending on the automobile type. As a result, even when the output voltage from the vehicle height sensor stays the same, the converted vehicle height value differs. The drawing shows schematically respective characteristics C1, C2, C3, for example, of a C1 type vehicle, a C2 type vehicle, and a C3 type vehicle and shows that vehicle heights h1, h2, h3—based on the same output voltage v1 of the vehicle height sensor—differ.
According to the foregoing initialization technique, if the design reference value is not stored previously, it is not possible to detect the vehicle height. Therefore, in some automobile assembly factories, as shown by a flowchart in FIG. 4(c), it is necessary to store a design reference value to ECU of the automobile (S301), set the automobile on a level block and detect an output of a vehicle height sensor as a detected output (S302), and calculate a vehicle height by calculating a detected output voltage based on the design reference value in ECU and storing the calculated vehicle height as the reference vehicle height (S303). In the foregoing technique, at step S302, it is necessary to install a level block having a space in correspondence with one automobile, a step stage having the space is to be ensured in an assembling factory, and a stage for carrying out step S301 is to be arranged on an upstream side of the level block of a fabricating line. Such requirements constitute a restriction in designing the factory layout and restrict the degree of freedom of design. Further, at the assembly factory, the design reference value for the automobile to be assembled must be stored while moving an apparatus along the fabrication line for carrying out step S301 so as to improve the manufacturing efficiency.